A method of metal container manufacture in current use by the metal container industry is the two-piece can process. This process involves forming a drawn cup from a metal sheet and then deep drawing the cup into a can configuration. After the can body is completely configurated and decorated, but before the end is assembled onto the body, the interior surface of the can body is coated with a protective coating of a synthetic resin material.
It is conventional practice to apply, as the coating material, a heat hardenable resin dispersed in an aqueous medium which is sprayed into the interior walls of the container. The open-ended can is caused to be passed through an internal coating station, commonly referred to as a "coating tunnel," where the coating liquid is sprayed into the interior of the can to coat its internal surface. The wet-coated container is then passed through an oven in which hot air is circulated to evaporate the aqueous medium and harden the coating.
In some conventional systems, the coating is sprayed into the interior of the container while the container is inverted in an upright position on a reticulated belt. In such systems, as the containers travel through the coating tunnel, the interior surface walls of the containers are contacted with an aqueous dispersion of a coating resin by spraying means, usually comprising a plurality of single orifice nozzles disposed under and along the reticulated belt, which direct a sequence of wide atomized sprays of wet resin coating onto the interior walls.
Substantially all the conventional nozzles used in such applications are of the single orifice type which make controlling the application of the coating to the interior surface difficult, especially near the open end of the container.
The airless nozzle most commonly used today in such applications includes an internal, hemispherical passage termination which is cut through by an external, V-shaped groove to form an elongated, elliptical-like orifice. Liquid material pumped at high pressures through such a spray nozzle is forced by the hemispherical termination of the passageway to converge in its flow at and through the elongated orifice. Because of the converging flow at the orifice, the liquid material is expelled through the orifice into a planar, expanding, fan-like film which breaks into spray particles which are carried by their momentum to the article target.
In the prior art, it was common during container manufacturing operations to simply excessively coat the interior of a container so that the "hard-to-reach" areas would receive a sufficiently thick layer of coating deposited thereon. This, naturally, left an excessively thick layer of coating in the "easy-to-reach" areas of the container. One attempt to cure this problem is disclosed by Stumphauzer, U.S. Pat. No. 3,697,313. The procedure of Stumphauzer results in a substantially uniform coating being applied over the interior surface of the sidewall of the container.
A further attempt to remedy this problem is disclosed by Rehman, U.S. Pat. No. 4,378,386. Rehman relates to a method and apparatus for applying a uniform coating to the interior surface of a container utilizing at least two nozzle means to apply the coating material. One of the nozzle means is operative to spray the lower portion of the cylindrical sidewall of the container while the other nozzle means is operative to spray the top portion of the sidewall and the crown or center section of the bottom wall. Rehman is especially intended for use with high solids or higher solids liquid coating materials. It is a specific object of Rehman to achieve a very even and uniform coating of the interior of the container.
Such prior systems are commonly plagued by excessive overspray and nozzle misting, each of which is costly due to the coating liquid that is wasted during such operations. It is more cost efficient to place more coating liquid on the interior areas where it is required most, that is, near the top and/or the bottom, particularly the bottom, of the container and less on the middle of the sidewall of the container as the top of the container is worked upon during the necking operation and the bottom of the container is in contact with the pin tip of the pin chain. The middle sidewall normally has minimal contact and consequently needs less coating.
Thus, there has developed in the metallic container manufacturing industry a need for an internal coating nozzle assembly capable of placing the coating where it is most required within the interior of the container, while also minimizing the amount of wasted coating liquid normally produced by an internal coating operation.